Nonlinear optical crystal

ABSTRACT

There is provided a nonlinear optical crystal which is presented by the formula: K 2 Al 2 B 2 O 7 . This nonlinear optical crystal is a vacuum ultraviolet light generating nonlinear optical crystal which is easy to grow and of high practical use. There are also provided a wavelength conversion method using this crystal, and an element and a wavelength conversion apparatus for use in the method.

This application is a 371 application of PCT/JP99/04199 filed Aug. 4,1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonlinear optical crystal. Morespecifically, the invention relates to a novel nonlinear optical crystaluseful as a wavelength conversion crystal for generating vacuumultraviolet light or the like, and a wavelength conversion method usingthe wavelength conversion crystal, as well as an element and awavelength conversion apparatus for use in the method.

2. Background Art

With the development of laser technology, it has become an importancesubject to realize solid-state lasers having performance which allowsfor the applications of laser technology. One such subject is to putinto practice all solid-state vacuum ultraviolet laser light sources ofshorter wavelength.

To realize an entirely solid-state vacuum ultraviolet laser light sourceof short wavelength, there is a need for a nonlinear optical crystalwhich has a double refraction index of about 0.07 and an absorption edgewhich lies in the range of short wavelengths of 150–160 nm. As prior artnonlinear optical crystals which satisfy these characteristics, thefollowing ones have been known:Sr₂Be₂B₂O₇ (SBBO),KBe₂BO₃F₂.

These publicly known prior art SBBO and KBBF, however, have the largeproblem that both crystals are difficult to obtain, because they areextremely difficult to grow.

SUMMARY OF THE INVENTION

Therefore, the invention provides a nonlinear optical crystal. Morespecifically, the invention provides a novel nonlinear optical crystalfor an entirely solid-state generation of vacuum ultraviolet light,which has the required characteristics and is easy to obtain throughcrystal growth instead of the prior art SBBO and KBBF, and a wavelengthconversion method using such novel nonlinear optical crystal, as well asan element and a wavelength conversion apparatus for use in the method.

To solve the above-described subject, the invention provides a nonlinearoptical crystal represented by K₂Al₂B₂O₇, and a wavelength conversionmethod using this nonlinear optical crystal, as well as an element and awavelength conversion apparatus for use in the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily appreciated and understood fromthe following detailed description of a preferred embodiment of theinvention when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a cross-sectional view of the construction of a growingfurnace used in an embodiment of the present invention;

FIG. 2 is a view showing a result of X-ray diffraction of the structureof a KAB crystal according to the invention; and

FIG. 3 is another a view showing another result of X-ray diffractionsimilar to that shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention will be described below with reference toa nonlinear optical crystal represented by K₂Al₂B₂O₇ (referred to simplyas the KAB crystal) which is provided according to the invention. TheKAB crystal has a structure in which K and Al are substituted for therespective Sr and Be sites of the publicly known SBBO crystal, i.e.,Sr₂Be₂B₂O₇, although there is a difference in electric charge betweenboth crystals.

The KAB crystal of this invention has a double refraction index of about0.07 which is a nature approximately equal to that of the publicly knownSBBO crystal. Accordingly, the KAB crystal is expected to generatevacuum ultraviolet light. The KAB crystal can easily be grown by amethod such as a flux method.

The flux method is one kind of solution growth method, and ischaracterized by TSSG (Top Seeded Solution Growth), i.e., the process inwhich a seed crystal attached to a rotating shaft is immersedimmediately below the surface of a solution to increase the degree ofsupersaturation by means of a decrease in temperature, thereby growing acrystal. In addition, the flux method is characterized by melting a fluxand a source material.

Since the melting point of the KAB crystal is high, it is morepreferable to grow the KAB crystal by the flux method (solution growthmethod) than by a melt method (melt growth method).

In this flux method, the crystal growth can be made far easier by usinga flux such as lead oxide, sodium fluoride (NaF) cesium fluoride (CsF),lead fluoride or potassium chloride.

Accordingly, the KAB crystal of the invention is easy to grow and issuperior in practical terms, and is extremely useful as a practicalnonlinear optical crystal for generating vacuum ultraviolet light.

This crystal is actually utilized as an element for wavelengthconversion or a wavelength conversion apparatus incorporated in thiselement.

Incidentally, it goes without saying that inevitable trace elements areallowed to be inevitably incorporated into the composition of thecrystal of the invention by a growth process or a source material.

This invention will be described below in further detail with referenceto an example.

EXAMPLE

Source materials having the following compositions were used to grow acrystal in the growing furnace shown in FIG. 1 by way of example:K₂CO₃ (34 mol %)Al₂O₃ (19 Mol %)B₂O₃ (45 mol %)KCl (2 mol %).

The growing furnace shown in FIG. 1 has a construction like acylindrical resistance heating furnace. In this furnace, its heater isvertically divided into five layers each of which can be independentlycontrolled. A temperature program setting device capable of controllingtemperature in units of a minimum of 0.1° C. is used as a control partfor the heater, and a quartz tube is disposed between the heater and acrucible so that a steep temperature gradient near the crucible isrestrained. The crucible is made of platinum, and is arranged to move upand down by an elevating device lying at the bottom of the furnace, sothat the crucible can be charged with a source material in a heatedstate. In addition, in order to correct a change in the temperature ofthe solution surface, a solution surface heater is disposed to prevent adecrease in temperature due to evaporation near the solution surface,thereby providing a temperature distribution optimum for crystal growth.At a temperature of about 1,000° C., the source material was melted inthe atmospheric air, and was then cooled to grow into a microcrystal.The rate of temperature decrease was 0.2–0.3° C./day, and the speed ofrotation was 30 rpm (the direction of rotation was reversed at intervalsof 3 minutes).

Through the above-described growth, a crystal of size about 3 mm wasobtained.

The result of an analysis using plasma emission spectrometry (ICP)showed that this crystal had the composition of K₂Al₂B₂O₇. As isapparent from the result of four circle X-ray diffractometry shown inFIGS. 2 and 3, it was confirmed that the structure of the obtainedcrystal was similar to that of the SBBO crystal but K and Al were 100%substituted for its Sr and Be sites, respectively.

In the evaluation of wavelength conversion characteristic (nonlinearity)of the crystal, when the crystal was illuminated with the fundamentallight (wavelength 1,064 nm) of a Nd:YAG laser, the occurrence of lightof second harmonic (532 nm) was confirmed.

In addition, when the double refraction index of this crystal wasmeasured by an oil immersion method, it was confirmed that the value was0.07 and was approximately equal to that of the SBBO crystal.

It is to be noted that since the shortest SHG wavelength of the priorart KBBF crystal is 185 nm or less and that of the prior art SBBOcrystal is 200 nm or less, the KAB crystal of this invention can bephase-matched to approximately 200 nm. The absorption edge of the KABcrystal was 180 nm or less.

In addition, the growth of the KAB crystal of this invention is far moreeasy and far more efficient compared to the case of growth of SBBO andKBBF.

Incidentally, the Vickers hardness of the grown KAB crystal was about300, and from the result of a water resistance test using immersion atroom temperature, it was confirmed that the KAB crystal did not melteven after the passage of ten days or more.

In accordance with the invention, there is provided a K₂Al₂B₂O₇ (KAB)crystal as a vacuum ultraviolet light generating nonlinear opticalcrystal which is easy to grow and of high practical use, and awavelength conversion method using this crystal, as well as an elementand a wavelength conversion apparatus for use in the method.

1. A method of making a nonlinear optical crystal for generatingultraviolet light, said nonlinear optical crystal comprising a compoundrepresented by the formula K₂Al₂B₂O₇, said method comprising growing anonlinear optical crystal comprising a compound represented by theformula K₂Al₂B₂O₇ via solution growth with a flux that is at least onematerial selected from the group consisting of lead oxide, sodiumfluoride, cesium fluoride, lead fluoride or potassium chloride.
 2. Amethod of converting a wavelength for generating ultraviolet light saidmethod comprising: growing a nonlinear optical crystal comprising acompound represented by the formula K₂Al₂B₂O₇ via solution growth with aflux, and illuminating, with laser light, a nonlinear optical crystalcomprising a compound represented by the formula K₂Al₂B₂O₇.
 3. A methodof converting a wavelength according to claim 2, wherein said growingcomprises growing by solution growth with a flux that is at least onematerial selected from the group consisting of lead oxide, sodiumfluoride, cesium fluoride, lead fluoride or potassium chloride.
 4. Awavelength conversion element for generating ultraviolet light, saidwavelength conversion element comprising: a nonlinear optical crystalcomprising a compound represented by the formula K₂Al₂B₂O₇, wherein saidnonlinear optical crystal has an input surface capable of receivinginput laser light having a fundamental wavelength, and wherein saidnonlinear optical crystal has an output surface capable of transmittingan output laser light having a second harmonic.
 5. A wavelengthconversion element according to claim 4 wherein said nonlinear opticalcrystal comprises a crystal grown via solution growth with a flux.
 6. Awavelength conversion element according to claim 5, wherein said growingcomprises growing by solution growth with a flux that is at least onematerial selected from the group consisting of lead oxide, sodiumfluoride, cesium fluoride, lead fluoride or potassium chloride.
 7. Awavelength conversion apparatus for generating ultraviolet light, saidwavelength conversion apparatus comprising: a wavelength conversionelement comprising a nonlinear optical crystal comprising a compoundrepresented by the formula K₂Al₂B₂O₇, wherein said nonlinear opticalcrystal has an input surface capable of receiving input laser lighthaving a fundamental wavelength, and wherein said nonlinear opticalcrystal has an output surface capable of transmitting an output laserlight having a second harmonic.
 8. A wavelength conversion apparatusaccording to claim 7, wherein said nonlinear optical crystal comprises acrystal grown via solution growth with a flux.
 9. A wavelengthconversion apparatus according to claim 8, wherein said growingcomprises growing by solution growth with a flux that is at least onematerial selected from the group consisting of lead oxide, sodiumfluoride, cesium fluoride, lead fluoride or potassium chloride.
 10. Awavelength conversion method for generating ultraviolet light, saidwavelength conversion method comprising: illuminating, with laser light,a nonlinear optical crystal comprising a compound represented by theformula K₂Al₂B₂O₇.